Re: Focus shift (was: Medium format and plastic sheets -Rep

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 11 Jul 97 at 9:43, Timothy E. Coffer wrote:

> I can also jump on the "no focus correction for EIR" bandwagon. I have
> plenty of images taken with an autofocus camera (Nikon 8008s) to prove
> it. And the comment relayed from Kodak that "visible light predominates"
> only confirms my opinion that this film (EIR) is primarily a false-color film.
> Not that I have a problem with that "limitation" at all. One test is to shoot a
> subject of known infrared luminosity with both film. HIE clearly reacts to
> the reflectivity in the IR spectrum, whereas EIR seems more involved
> with shifting the painter's palette around. 
> Now what pigment layer in IER's emusion is supposed to be the
> IR-sensitive one again? :=)

Here an attempt to clear up the fog (pun intended)....

AFAIK HIE and EIR are almost equally sensitive to infrared, *BUT* 
keep in mind that sensitivity of HIE drops at least 5 stops when 
using a #87C, and at least one or two stops more when using a #87B.
We are talking 200 ASA vs 10 or less ASA....a world of difference if 
you mix those spectra in one image....you hardly record any 
substantial amount of IR, definately not in the spectrum 850-950nm. 
And the Wood effect already starts at 695nm....
The only reason I can image why EIR does not require this focus shift 
is this different spectral 'treatment'....
Note that this still doesn't rule out the possibility of getting 
sharper images with an APO lens....and by comparing it with 
closing down you run the risk of substituting focus shift with 
diffraction, especially for that part of the spectrum....
Guenter Spitzing makes a hard statement in his book about this, not 
just some ponderings....

Also, the report about the sharp shots with a Rollei 80/2.8 at f5.6 
is only useful if they are also taken wide open....the IR-dot might 
as well lay inside/close to the DOF-range at f5.6....;-))

Odd detail: AFAIK, the Leica 180/3.4 APO was developed for the
Canadian army....one wonders what arguments they had in mind related 
to IR, both b&w and color....
  
PS: I just saw that Zeiss also made a 200/2.0 for Contax....too bad 
there are no adaptors for Contax->EOS....OTOH, it is better for my 
wallet....;-))


- -- 
Bye,

Willem-Jan Markerink


      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]
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